Towards a precise measurement of the cosmic-ray positron fraction

This thesis deals with detector concepts aiming at a precise measurement of the cosmic-ray positron fraction extending to an as yet unreached range of energy. The indirect search for dark matter is the main motivation for this endeavour. The evolution and large-scale structure of the Universe is described by the Hot Big Bang model which is well supported by observational evidence. In this model, roughly a quarter of the energy density of the Universe must be made up of the elusive dark matter. Evidence for its existence comes from, for example, the observation of galactic rotation curves, dynamics of clusters of galaxies, and the pattern of anisotropies in the cosmic microwave background. While the nature of dark matter is as yet unknown, the most popular candidate for its constituents is the neutralino included in supersymmetric extensions to the standard model of particle physics. Neutralinos annihilating in the Galactic halo are considered as a potential primary source of cosmic-ray positrons. For the calculation of the expected secondary background of positrons, a common cosmic-ray propagation model has been adopted and its uncertainties have been assessed. The cosmic-ray positron fraction data available so far indicate an excess over the expectation for purely secondary production, a trend recently confirmed and intensified by measurements of the PAMELA satellite detector. The AMS-02 detector will be ready for installation on the International Space Station in 2010 and is designed to perform precision spectroscopy of many different cosmic-ray species including positrons. Here, a design concept for a new detector, called Positron Electron Balloon Spectrometer (PEBS), is presented. Intended for a measurement of the cosmic-ray positron fraction on one or more flights at high altitude using a long-duration balloon, PEBS will have an unprecedentedly high acceptance of almost 0.4m^2sr. A first launch could take place in 2012. Using a superconducting magnet to create a mean magnetic field of 0.8T and a scintillating fibre tracker with silicon photomultiplier readout, it will allow reliable charge-sign and momentum measurements up to at least 100GeV. The enormous challenge of reliably identifying positrons in front of the vast proton background is tackled by a combination of two independent subdetectors for particle identification. The first one is an electromagnetic calorimeter which will consist of layers of tungsten absorber interleaved with scintillator bars, read out by silicon photomultipliers. The second one is a transition radiation detector (TRD) similar to the one built for AMS-02, made of an irregular fleece radiator followed by thin-walled detection tubes. A detailed Monte Carlo simulation of PEBS, based on Geant4,was created to study the expected performance of the detector, along with a reconstruction and analysis suite. The simulation predicts a momentum resolution of 18% for 100GeV positrons. For the same energy, the calorimeter is predicted to have an energy resolution of 6% and a proton rejection of 3000 at 80% positron efficiency. The transition radiation detector will provide an additional rejection factor of 700, again with 80% positron efficiency. Using testbeam data acquired with a prototype for the AMS-02-TRD, the accuracy of the simulation of transition radiation and ionisation losses provided by Geant4 was studied. Excellent agreement was found between the transition radiation spectra in data and simulation. Small discrepancies at the 25%-level are present in the tails of the proton energy loss spectra but this makes the predicted proton rejections uncertain by a factor of two. In a series of testbeam measurements, the proof of principle was established for the scintillating fibre tracker with silicon photomultiplier readout. The intrinsic spatial resolution achieved at the current level of design is 0.07mm. Neutralino dark matter was studied in the minimal supergravity grand unification (mSUGRA) model. Assuming that neutralino annihilations are enhanced by boost factors taken from best fits to the positron fraction data published so far, both PEBS and AMS-02 will be capable of substantially constraining mSUGRA parameter space. It is shown that a moderately good fit to the high-energy PAMELA data can be obtained in the mSUGRA model as well. At the same time, the low-energy PAMELA data may hint at charge-sign dependent solarmodulation effects.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2009

Author dc:creator

• Gast, Henning

Contributors dc:contributor

• Schael, Stefan

• info:eu-repo/classification/ddc/530
• Kosmische Strahlung
• Dunkle Materie
• Magnetisches Spektrometer
• Physik
• cosmic rays
• dark matter
• magnet spectrometer

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-113344%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A50818